1. Technical Field
The present invention relates to an antenna duplexer having a plurality of filters.
2. Background Art
In recent years, a cellular phone of a communication system of performing transmission/reception simultaneous communication such as W-CDMA (Wideband Code Division Multiple Access) is being rapidly spread. Accordingly, demand of an antenna duplexer such as a duplexer is increasing. As elements constructing an antenna duplexer, an SAW (Surface Acoustic Wave) element, a boundary elastic wave element, a BAW (Bulk Acoustic wave) element, and the like which are excellent from the viewpoints of small size, low height, and mass production are mainstream.
Generally, an antenna duplexer has two filters (a transmission filter and a reception filter) to separate a signal in a transmission band and a signal in a reception band neighboring the high frequency side of the transmission band from each other. Particularly, as the transmission filter, a ladder filter in which series resonators and parallel resonators are connected in a ladder shape is employed.
For example, the gap (crossband) between the transmission band and the reception band in Band2 determined in 3GPP (3rd Generation Partnership Project) is 20 MHz (1.06% in expression of fractional bandwidth). The gap is narrower as compared with 20 MHz (fractional bandwidth: 2.36%) as the crossband of Band5 often used in conventional antenna duplexers.
Therefore, a technique of adding weight on an IDT (Inter Digital Transducer) of a resonator of a transmission filter in order to assure steepness to address the narrow crossband is proposed (refer to, for example, Japanese Translation of PCT Application No. 2001-500697).
Various techniques of making the propagation angles of the main elastic waves of resonators in a ladder filter are also proposed (refer to, for example, Japanese Unexamined Patent Publication No. H07-283688 and WO 2005/060094).
However, Japanese Translation of PCT Application No. 2001-500697 relates to the technique for narrowing the bandwidth and does not disclose means realizing an antenna duplexer whose bandwidth is wide like 60 MHz in Band2. That is, the transmission filter in the antenna duplexer disclosed in Japanese Translation of PCT Application No. 2001-500697 has a problem such that when steepness is increased to sufficiently assure attenuation in the reception band, the transmission bandwidth is narrowed, and a loss in a wide passband increases. Particularly, the fractional bandwidth of the transmission/reception passband in Band2 is 3.2% and wide, and it is very difficult to maintain a small loss in the wide transmission passband.
It is expected that, in future, not only in Band2 but also Band3, Band8, and the like, an antenna duplexer capable of satisfying both a low loss characteristic in a wide band and a steep attenuation characteristic is in demand.
Conventionally known techniques, however, have a problem such that when a low loss characteristic is realized in a wide band, sufficient steepness cannot be obtained.